The invention is related to a drawing-off process or a molten material such as glass contained in a crucible, the walls and bottom of which are at least partly cooled.
The invention also relates to a drawing-off device making use of this process.
The invention is applicable to drawing off all molten materials contained in a crucible, for which the walls and the bottom are at least partially cooled. One specific application is the vitrification of very high activity nuclear waste.
It is well known that a material such as glass can be melted by adding this material into a crucible in the solid state and then melting it in a crucible, for example by induction.
It is also known how to at least partially cool the walls and the bottom of this crucible, in order to increase the life of the crucible by keeping it at a relatively low temperature. In particular, this cooling can be achieved by water circulation. Its effect is to form a layer of stratified solid glass at the contact with the cooled walls and the bottom of the crucible, isolating the bottom and the walls from the molten material. It is thus possible to produce molten materials such as glass at high temperatures (more than 1150xc2x0 C.) without replacing the crucible too frequently.
When this known technique is used for vitrification of very high activity nuclear waste, the waste is added into the melting glass contained in the crucible. The weight of glass containing the waste is then emptied into a container located under the crucible, through a drawing off device provided for this purpose in the bottom of the crucible.
The life of the crucibles is increased by cooling the walls and the bottom of the crucibles, and this increase is particularly advantageous in this application. Used crucibles form radioactive waste that must be conditioned and stored.
Furthermore, when crucibles with cooled walls and bottoms are used for the vitrification of nuclear waste, they form waste with a lower degree of activity than uncooled crucibles. Melting glass contracts on contact with the cold walls, such that the solidified glass does not adhere to the walls. Consequently, the crucible may be perfectly cleaned at the end of the life, unlike an uncooled crucible in which there are always encrusted highly radioactive solid glass particles, even after cleaning.
Different emptying techniques are known for drawing off the molten material contained in a crucible.
According to a first technique, a pour nozzle is used that passes vertically through the bottom of the crucible and extends over a certain length below the crucible. The nozzle is cooled to form a glass plug inside the nozzle that normally prevents the crucible from being emptied. When it is desired to draw the liquid off, the nozzle is heated using an auxiliary heating system; for example induction.
There are several disadvantages of this emptying technique:
the pour nozzles are consumable wear parts with a short life;
when the nozzle is heated for drawing off purposes, the plug drops as a single piece and causes splashes of liquid glass, which is not a good thing when the glass contains highly radioactive waste;
the pour nozzle is only cooled very slowly, such that it is impossible to control the drawing off flux and to stop pours very sharply.
Another emptying technique in particular is described in document FR-A-2 704 634.
In this case, the drawing off device comprises a circular pour orifice passing through the cooled bottom of the crucible, a cooled slide valve capable of closing and opening this orifice in a controlled manner, and a metal sleeve (also circular) inserted in the pour orifice. The temperature of the metallic sleeve in this device, usually made of molybdenum, is different from the temperature of the bottom of the crucible and it projects upwards inside the crucible, so as to pass through the solidified glass layer contained in the bottom of the crucible to penetrate into the molten glass.
When the valve slide is closed, a solid glass plug is formed in the metallic sleeve, in contact with the cooled upper surface of the said slide. Therefore, the valve is isolated from the melting glass, such that its life is approximately the same as the life of the crucible and that it can be cleaned equally efficiently.
The valve slide is opened when it is desired to draw off. Since the metallic sleeve and the solid glass plug formed in the sleeve are no longer in contact with the cooled slide, the viscosity of the hot glass in contact with the sleeve reduces to eliminate the solid glass which drops out by gravity. The position of the valve slide is then a means of adjusting the liquid glass flux by more or less closing the pour orifice.
This pour device also has some disadvantages.
A first disadvantage relates to oxidation of the pour sleeve. It is found that the sleeve faces in contact with air at the end of emptying are oxidized. This reduces the life of the sleeve, which is contrary to the required purpose using a cooled crucible, which is precisely to guarantee an increase in the life of the crucible.
Another disadvantage of the drawing off device described in document FR-A-2 704 634 consists of permanent retention of the glass at the end of emptying. Since the sleeve projects upwards over a certain distance from the bottom of the crucible, emptying is not complete. This can cause problems, particularly if the molten material contained in the crucible is glass with a high content of platinoids. If these elements are not put back into suspension, they will settle to the bottom of the crucible. Eventually, this can cause electrical malfunctions such as the occurrence of electrical arcs that can cause local melting of the upper wall of the double wall structure forming the bottom of the crucible. This melting can lead to penetration of the said upper wall, which causes leaks of the cooling water in the crucible.
In this case, the only solution is to eliminate the sleeve placed in the pour orifice. However, there is then a risk that drawing off will be impossible. Thus, when the molten material is a fairly bad conductor of heat and has a particularly high melting point, as is the case particularly for glass with a high content of platinoids, the glass hardly ever starts pouring when the valve is opened. The glass plug is then deformed under the effect of heating and the glass content in the crucible, before moving into an equilibrium position that prevents the valve slide from being closed.
The purpose of the invention is precisely to describe a process and a device for drawing off a molten material such as glass contained in a crucible, designed not to have the disadvantages of known drawing off techniques, and particularly so that pour starting and stopping times and the flux rate can be precisely controlled using parts with a life comparable to the life of a cold crucible, while providing good reproducibility of the pour process, avoiding glass splashes and providing good control and good stability of the melting glass jet, regardless of the nature of the jet, without any risk of glass retention at the end of emptying.
According to the invention, this result is achieved using a drawing off process of a molten material contained in a crucible with an at least partly cooled bottom, to form a solidified layer of material at the contact with the said bottom, process according to which drawing off is achieved by opening a valve, also cooled, initially closing off a pour orifice formed in the bottom of the crucible, characterised in that the shape of the pour orifice as seen from above is elongated, and that drawing off is started at a first end of the said orifice forming a thermal bridge between a part of the bottom delimiting the said first end and the molten material contained in the crucible above the said solidified layer.
Thus, by replacing the usually circular shape of the pour orifice by an elongated shape and forming a thermal bridge between one of the ends of this orifice and the molten material, there is no doubt that it will be possible to start the pouring of the material, regardless of its nature, without any risk of this material being retained at the end of emptying.
Furthermore, due to the fact that the thermal bridge is made entirely within the crucible, it is never in contact with air such that risks of oxidation are reduced. Therefore its life is the same as the life of the entire crucible.
Furthermore, the layout of the thermal bridge at one end of an elongated pour orifice is sufficient to control tripping of the pour in a perfectly controlled and reproducible manner. In particular, the plug formed by the material is gradually melted starting from this end, such that there is no risk of projection due to the solid plug dropping, unlike the situation with all existing techniques.
Advantageously, the thermal bridge is formed by placing a bar made of a thermally conducting material in the crucible in contact with the bottom part of the crucible along the first end of the pour orifice. However, the temperature of the poured glass is limited to the melting temperature of the metal making up the thermal bridge.
In one preferred embodiment of the invention, a valve is used comprising a cooled slide that slides under the pour orifice along a longitudinal axis of this orifice.
Preferably, the drawing off position is on the longitudinal axis of the pour orifice, making the shape of the first end of the pour orifice convex and the end near the cooled slide concave, centred on the longitudinal axis of the pour orifice. Thus, the first end of the pour orifice is advantageously approximately in the shape of a V.
In the preferred embodiment of the invention, a crucible is also used with a relatively thin bottom around the pour orifice. This characteristic facilitates melting of the plug formed in the pour orifice after the valve has opened.
For the same reason, the bottom of the crucible advantageously does not have any cooling means in the immediate vicinity of the pour orifice.
Another purpose of the invention is a device for drawing off a molten material contained in a crucible comprising a bottom and means of at least partly cooling this crucible to form a solidified layer of the material in contact with the bottom, device comprising a valve, means of cooling this valve, and a pour orifice formed in the bottom of the crucible and normally closed by the valve, characterised in that the shape of the pour orifice as seen from above is elongated and in that the means forming the thermal bridge are inserted between the part of the bottom delimiting a first end of the pour orifice and the molten material contained in the crucible, above the said stratified layer.